Frequently Asked Questions 

Q? We have recently added Rumensin to our dairy cow grain in a tie-stall barn. Although milk production has increased, milk fat has dropped 0.6% and protein dropped 0.2%. What is going on??

A: Due to the anti-microbial impact of monensin on Gram positive bacteria in the rumen, it clearly depresses butterfat. This has been consistently shown in the research and in fact, on the label, there is a statement that informs the user that "You may notice reduced milk fat percentage".

The main reason for the butterfat depression is that Monensin inhibits hydrogen producing bacteria eg. cellulolytic ruminococci. This explains the decrease in methane production and also explains the decrease in cellulose digestion (and feed intake). This negative effect of monensin on fiber digestion has the consequence of depressing milk fat percentage. In the Elanco literature you will see a review of nine milk trials across North America (involving 818 cows) that showed that milk fat is depressed up to 0.3% with an increase in the monensin concentration in the diet.

In addition, the label indicates that it should be fed in a TMR (total mixed ration), a 'complete feed' containing 11 to 22 grams per ton of monensin on a 100% dry matter basis. A copy of the label is available at http://www.elanco.com/us/pdf/rum80.pdf

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